1. Field of the Invention
The invention relates to methods and systems for treating vehicle exhaust gas to provide reduced vehicle tailpipe emissions.
2. Background Art
The operation of a typical internal combustion engine, as may be found in motor vehicles, results in the generation of engine exhaust which includes a variety of constituents, including carbon monoxide (CO), hydrocarbons (HC), and nitrogen oxides (NOx). The rates at which an engine generates these constituents are dependent upon a variety of factors, such as engine operating speed and load, engine temperature, spark timing, and EGR. Moreover, such engines often generate increased levels of one or more constituents, such as NOx, when the engine is operated in a lean-burn cycle, i.e., when engine operation includes engine operating conditions characterized by a ratio of intake air to injected fuel that is greater than the stoichiometric air-fuel ratio, for example, to achieve greater vehicle fuel economy.
Vehicle exhaust treatment systems often employ a three-way catalyst, referred to as an emission control device, disposed in an exhaust passage to store and release constituents such as NOx depending upon engine operating conditions. For example, U.S. Pat. No. 5,437,153 teaches an emission control device which stores exhaust gas NOx when the exhaust gas is lean, and releases previously-stored NOx when the exhaust gas is either stoichiometric or xe2x80x9crichxe2x80x9d of stoichiometric, i.e., when the ratio of intake air to injected fuel is at or below the stoichiometric air-fuel ratio. Such systems often employ open-loop control of device storage and release times (also respectively known as device xe2x80x9cfillxe2x80x9d and xe2x80x9cpurgexe2x80x9d times) so as to maximize the benefits of increased fuel efficiency obtained through lean engine operation without concomitantly increasing tailpipe emissions as the device becomes xe2x80x9cfilled.xe2x80x9d Thus, for example, U.S. Pat. No. 5,437,153 teaches an open-loop method for determining appropriate device fill times wherein an accumulated estimate of instantaneous engine-generated NOx (all of which is presumed to be stored in the device when operating in a linear operating range) is compared to a reference value representative of the instantaneous maximum NOx-storing capacity of the device, determined as a function of instantaneous device temperature. When the accumulated estimate exceeds the reference value, the xe2x80x9cfillxe2x80x9d is deemed to be complete, and lean engine operation is immediately discontinued in favor of an open-loop purge whose duration is similarly based on the estimated amount of stored NOx.
Unfortunately, such open-loop, accumulator-based models generally fail to address the fact that the instantaneous storage efficiency of the device is a complex function of many variables, including sulfur poisoning, the temperature of the device, device oxygen storage, component aging, and vehicle driving conditions. Accordingly, the estimate of the amount of a constituent exhaust gas, such as NOx, which has been stored in the device is only an approximation and, hence, any purge time determined on the basis of the estimated stored amount is equally susceptible to error.
The inventors herein have recognized a need for a method and system for purging an emission control device which does not solely rely upon a prior estimate, obtained during a device fill, of the amount of a constituent exhaust gas which must be released from the device during the purge event.
Under the invention, a method is provided for releasing, from an emission control device receiving exhaust gas generated by an internal combustion engine, an amount of a constituent of the exhaust gas that is stored in the device. The method includes generating, during a first time period, a first flow of exhaust gas through the device that is rich of a stoichiometric air-fuel ratio, for example, by operating the engine at a rich operating condition characterized by a ratio of intake air to injected fuel that is greater than the stoichiometric air-fuel ratio; detecting, as with a suitable sensor, a characteristic of the first flow of exhaust gas at a position within the device between an upstream portion of the device and a downstream portion of the device; and determining the first time period on the basis of the detected characteristic. By way of example, in a preferred embodiment, the sensor is a xe2x80x9cswitching-typexe2x80x9d oxygen sensor whose output signal is representative of a concentration of oxygen in the exhaust gas flowing through the device. Thus, in the preferred embodiment, the rich operating condition is selected; and hence, the rich air-fuel mixture is maintained to purge the device of stored gas, such as stored NOx, until the output signal generated by the oxygen sensor indicates that the oxygen concentration at the sensor""s position within the device has fallen below a predetermined reference value, thereby evidencing a xe2x80x9cbreaking throughxe2x80x9d of available hydrocarbons to the sensor""s relative position within the device. At this point, the upstream portion of the device is nearly completely purged of stored NOx, while the relatively smaller portion of the device downstream of the oxygen sensor still stores a quantity of NOx.
In order to substantially purge stored gas from the downstream portion of the device, the method preferably also includes calculating a first amount of fuel, in excess of the stoichiometric amount, necessary to purge substantially all of the stored gas remaining in the downstream portion of the device. Because a certain amount of rich exhaust gas has already been introduced into the exhaust system upstream of the device at the time the mid-device oxygen sensor output signal falls below the predetermined reference value, with the concentration of excess hydrocarbons in the introduced exhaust gas being a function of engine speed, load and air-fuel ratio during the period immediately preceding the xe2x80x9cswitchingxe2x80x9d of the sensor output, the method preferably also includes calculating a second amount of excess fuel that has already been introduced into the exhaust system upstream of the device.
The method further preferably includes subtracting the second excess fuel amount from the first excess fuel amount to obtain an additional excess fuel amount, and determining an additional time period, when operating with the rich air-fuel ratio at a current engine speed and load, necessary to supply the additional excess fuel amount for use in purging the downstream portion of the device. The rich operating condition is thus continued for a predetermined time period after the sensor output signal falls below the reference value, with the predetermined time period being no greater than the determined additional time period necessary to supply the additional excess fuel amount.
Thus, the invention advantageously only requires estimation of the amount of excess fuel required to purge the relatively small downstream portion of the device, and the concentration of excess hydrocarbons present in the exhaust gas during the rich operating condition. And, through careful adjustment of the size of the downstream brick, the amount of additional excess fuel necessary to substantially purge, or to otherwise release a desired amount of stored gas from, the downstream brick is preferably closely matched with the amount of excess fuel which has already been introduced into the exhaust system upstream of the device at the time the mid-device oxygen sensor switches, whereupon the additional time period can be reduced to a very small, preferably near-zero value for a nominal rich (purging) air-fuel mixture, and at a nominal engine speed and load. Thus optimized, any error in the resulting determination of the additional time period required during any given purge event to purge the downstream portion of the device is not likely to substantially impact the benefits conferred by the invention, which otherwise include an overall reduction in HC, CO and NOx emissions, and a significant improvement in vehicle fuel economy.
The above object and other objects, features and advantages of the present invention are readily apparent from the following detailed description of the best mode for carrying out the invention when taken in connection with the accompanying drawings.